I don't remember paying to become a developer or unlock my phone to load apps. Iirc its free to sign up. The cost is publishing and selling apps.
Developer Unlock is designed for Devs to load their Apps on devices for testing. As such it bypasses the store check. If you try and install a Lumia branded App from SD card onto the BLU, the store check causes it to fail due to "not available for this model" error. Developer Unlock is a workaround.
When I got my Dev account it cost me $20. I've never written an App in my life.